Magnetron type cold cathode ionization gauge having compression mounted cathode



Oct. 11, 1966 w. s. KREISMAN 3,

MAGNETRON TYPE COLD CATHODE IQNIZATION GAUGE HAVING COMPRESSION MOUNTED CATHODE Filed July 16, 1963 5 Sheets-Sheet 1 INVENTOR WALLACE S. KR EIS MAN ATTO R N EYS Oct. 11, 1966 w. s. KREIISMAN MAGNETRON TYPE COLD CATHODE IONIZATION GAUGE HAVING COMPRESSION MOUNTED CATHODE 5 Sheets-Sheet 2 Filed July 16, 1963 INVENTOR. WALLACE S. KREISIVIAN BY ATTORNEYS w mokomhwo .rzmmmDu mm 29 OF Oct. 11, 1966 w. s. KREISMAN MAGNETRON TYPE COLD CATHODE IONIZATIQN GAUGE HAVING COMPRESSION MOUNTED GATHODE 5 Sheets-Sheet 5 Filed July 16, 1963 FIG. 4

FIG. 5

ATTORN EYS United States Patent 3,278,786 MAGNETRON TYPE COLD CATHODE IONIZA- TION GAUGE HAVING COMPRESSION MOUNT- ED CATHODE Wallace S. Kreisman, Malden, Mass., assignor to GCA Corporation, Bedford, Mass., a corporation of Massachusetts Filed July 16, 1963, Ser. No. 295,300 Claims. (Cl. 313-157) This invention relates in general to precision instruments and in particular to an ultra-high vacuum cold cathode ionization gauge. The present application is a continuation-in-part of application Serial No. 157,372 filed December 6, 1961, now abandoned, on Ultra High Vacuum Cold Cathode Ionization Gauge.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 stat. 435; 42 U.S.C. 2457).

There are presently available on the market two basic types of cold cathode ionization gauges. One of these types utilizes a glass envelope within which metal electrodes are mounted. Its structure follows conventional lamp or tube design and relatively long metal leads extend to and provide support for the electrodes. With such leads, the achievement of good electrode alignment is quite difficult. Moreover, serious problems are encountered in many applications of such gauges. Shock and vibration can be destructive due to the fragility of the glass envelope and the relatively flimsy electrode support inherent in the design.

Obviously, a glass envelope has other unavoidable limitations. It has a relatively low melting point which limits the temperature at which it can be baked out to about 450 C. The glass most commonly used in the envelope is Pyrex, which has a relatively high helium permeation rate which limits the degree of vacuum that may be achieved with the envelope. Electrical shielding of the glass envelope is desirable, if not necessary, but it is not possible to shield the entire surface of the envelope. Finally, protection of the interior of the envelope from ultraviolet radiation is necessary and additional shielding must be provided for this function.

Although, as noted above, the concept of using a metallic rather than a glass envelope is not new, gauges now available with metallic envelopes usually are designed with metallic electrodes supported by flat ceramic rings or washers in the envelope. Typical of such structures is the gauge disclosed in the co-pending application of Kreisman entitled Cold Cathode Magnetron Type Ionization Gauge, Serial No. 149,906, filed November 3, 1961.

Many of the disadvantages of the glass envelope are avoided in such gauges with metallic envelopes. Nevertheless, there are other problems and limitations associated with fiat ceramic washers. First of all, the washers themselves must be custom-ground to very close tolerances attainable only at great expense. Because they are large in volume and surface area, outgassing of the washers is a long and difficult procedure. Their size poses still another problem because they seriously limit the open space available within the gauge and obstruct gas exchange betwen the discharge region of the gauge and the vacuum system to which it is attached when in use.

Provision of suitable support between the envelope wall and the electrodes requires that the inner portions of the washers be almost directly exposed to the active region of the gas discharge. Inevitably, washers so located become coated with a layer of metal sputtered from the cathode. The sputtered coating rapidly reduces the 3,278,786 Patented Oct. 11, 1966 'ice resistance normally provided by the non-conductive ceramic, and the operating lifetime of the gauge is thus shortened.

It is, therefore, a primary object of the present invention to improve ultra-high vacuum cold cathode ionization gauges.

A further object of this invention is to retain the advantages of a metallic envelope without the disadvantages and limitations of ceramic washer supports in an ionization gauge.

A further object of this invention is the simplification of design and reduction of cost of ionization gauges.

A still further object of this invent-ion is to increase the linearity and operating range, and to lengthen the lifetime of, cold cathode ionization gauges.

Another object of the present invention is to increase the sensitivity of cold cathodes by improving cathode configurations.

Still another object is .to increase the speed of response by improving the design and location of the anode of the gauge.

In general, the present invention resides in a unique electrode configuration and support within a metallic housing constituting a magnetron-type cold cathode ionization gauge. Feature-s of the invention include a spools'haped cathode having surfaces incline-d with respect to the magnetic field set up in the gauge. Moreover, the configuration of the cathode and the anode and their relative disposition cause the formation of a potential well when suitable voltages are applied to the electrodes. Electrons are caused by the potential well and the magnetic field to move and remain in a predetermined desired region within the gauge for extended periods. In this manner, the electrons are rendered more effective in ionizing gas molecules and creating positive ions, and the gauge is increased in sensitivity and linearity. Moreover, the initiation of a discharge in the gauge is facilitated and obtained more quickly, especially at very low pressure.

Several structural features of the gauge of the invention are also of great significance. The envelope is all metal providing complete shielding; it is at ground potential inhibiting surface leakage currentbetween anode and cathode of the gauge, and it is an integral part of the gauge, helping to initiate discharge and eliminating the need for a large magnet. The envelope is impermeable to helium and other rare gases; it is adaptable to fabrication from high permeability iron to permit the use of internal magnets; it is opaque, providing a shield against external ultra-violet radiation and soft X-ray radiation; and it is extremely rugged permitting its use in space vehicles and elsewhere in applications where breakage of the envelope could cause loss of pressurization of the vehicle.

The support for both anode and cathode of the gauge may be derived from thrust against those electrodes from the envelope walls. Such support may be had by utilizing insulating balls or spheres compressed between the inner envelope walls and the elements to be supported. The anode itself, in an area adjacent, but not interfering with, the support mechanism may be .apertured. Such an aperture is aligned with a tubulation by which the gauge is incorporated in a vacuum system to increase speed of response in the gauge.

For a beter understanding of the present invention together with other and further objects, advantages and features, reference should 'be made to the following detailed description which should be read in conjunction with the appended drawings, in which:

FIG. 1 is an exploded view of an embodiment of the cold cathode ionization gauge of the invention;

FIG. 2 is a view in vertical cross-section of the gauge illustrated in FIG. 1;

FIG. 3 is a top view of an alternative embodiment of the ionization gauge of the invention;

FIG. 4 is a view in vertical cross-section taken along the lines 4-4 of the gauge of FIG. 3; and

FIG. 5 is a vertical cross-sectional view taken along the lines 5-5 of the gauge of FIG. 3.

In FIGS. 1 and 2, there may be seen a generally cylindrical housing made preferably of seamless stainless steel tubing, which constitutes the envelope of the gauge. One end, the lower end as seen in the drawing, is closed, and the upper end of the housing 10 is formed with a relatively wide flange 12. A relatively large aperture 14 is formed at one point in the cylindrical wall surface, and two relatively small vertically aligned openings 16 and 18 are formed in the same wall at a point roughly opposite the opening 14.

Within the housing 10 a pair of outstanding ridges 20 and 22 are formed either by the machining of a central peripheral channel in the inside or by other means. The upper of these ridges, namely the ridge 20, is discontinuous for purposes which are explained in greater detail below. A tubulation 24 having a reduced shoulder 26 at one end and a sealing are-a 28 at the other end iits within the aperture 14, the shoulder 26 being of a diameter to provide a press fit into the aperture 14. The sealing area 28 is constituted of a metal having a coefficient of expansion matching that of a glass, which, in turn, may be incorporate-d int-o the system whose pressure is to be measured.

Support for an electrical connection to one of the electrodes of the gauge, in this case the anode, is provided by a cable end seal 30, through which a conductor 32 passes. The cable end seal is of such outside diameter that it closely fits the aperture 16 in which it may be sealed as by heliarc welding. The conductor 32 is welded to the stud 35 of the cable end seal which may be any one of several well-known commercially available devices. Ceramic-to-metal seals are provided between the conductor 32 and the tubular body 30. The seals serve to maintain the integrity of the vacuum ultimately drawn in the gauge, and they insulate the lead from the outer tubular body. Electrical connection to another of the internal electrodes of the gauge, here the cathode, is provided by a similar cable end seal 34, through which the conductor 36 passes.

A cylindrical member 37 forms the anode of the gauge, and it has four apertures or indentations 38, 42 and 44 formed in its wall at equally spaced points. It is not necessary that the apertures be formed through the anode wall. Instead, the wall may be spot-faced, indented 'or counterbored. Four spherical supports 46, 48, 50 and 52 made of any of several materials having good high voltage insulating qualities are utilized for anode support. The preferred supports are ceramic balls made, for example, of high density alumina. They are of slightly larger diameter than the apertures or indentations 38-44 in which they are lodged. In addition to the apertures or indentations which receive the spheres, a relatively large aperture 54 or a plurality of closely spaced small apertures may be formed in the wall of the sleeve '37 to align with the tubulation 24 for high speed gauge responses as explained below. In some instances, a fine mesh grid may be used to cover the large aperture to maintain anode potential over the aperture area Without impeding gas flow. At a point opposite the opening 54, an indentation may be formed in the outer cylindrical surface of the anode to receive a welding lug 56. When the gauge is assembled, the supporting spheres are held in place in the anode apertures or indentations and lowered into the housing 10. Theanode is then turned to force the spheres into the channel formed between the ridges 20 and 22 and to align the anode aperture 54 with the tubulation 24. The distance between the two ridges is somewhat smaller than the diameter of the spheres, and the anode ring is thus firmly looked under compression within the housing.

The cathode 60, which is generally spool-shaped with conically enlarged ends, is held in position coaxially of the anode in a fashion similar to that by which the anode is supported. A counterbored opening, visible in FIG. 2, is formed in the bottom Wall of the housing 10, and it receives a ceramic sphere 57 on which there is placed a cathode washer 58 having an extension for electrical connection. The washer may be press-fitted into the bottom of the cathode 60. Above the cathode 60, a ceramic ball 64 is compressed between the cathode opening and a counterbored opening 70, formed in a cover plate 72 to provide compression support for the cathode. The thrust exerted upon the cathode may be varied by variation of the size of the c-ounterbored openings in the top and bottom members. To prevent entrapment of gas, all support openings may be slotted as well as counterbored. 7 The cathode may be formed from a single piece, or it may be an assembly of parts such as a pair of washers, a pair of cones and a cylinder. Prior to assembling the cathode into the housing, the extension or lug on the washer 58 may be welded to the conductor 36 or to a flexible metal extension of that conductor. The other end of the conductor is welded to the external stud 39 on the cable end seal.

Reference specifically to FIG. 2 gives a clear picture of the method of assembly of the gauge. In the course of the assembly, as described above, the anode ring 37 is lowered into the envelope or housing 10, and the four spherical support balls are forced into the circumferential channel in the housing. At this time, and prior to the insertion of the cathode, electrical connection is made to the anode by welding the anode lug 56 to the free end of the anode conductor 3-2. The other end of the conductor 32, as noted above, is welded to the external stud 35.

Next, the cathode may be lowered into place. At this time, the cathode conductor 36 may be joined to the stud 39 of the cable end seal. Also, the extension of the cathode washer 58 and the conductor 36 may be joined directly or. through another thin metal strip by welding. The cable end seals and the tubulation are preferably heliarc welded into the housing, and the top member 72 may be similarly attached to the flange 12. Application of thrust to the cathode may require that the top member or plate 72 be bulged slightly outward.

In FIGS. 3, 4 and 5, a different and somewhat less expensive and less complicated gauge built in accordance with the invention is shown. Insofar as it is possible, similar parts are given reference numerals corresponding to those of FIGS. 1 and 2. In this instance, the housing 10 is also provided with three openings, into which the tubulation 24 and the cable end seals 30 and 34 are welded. The cathode is of the same shape as that of FIGS. 1 and 2, but it is made up less expensively of punched parts which are welded together. These parts consist of a piece of small diameter tubing 60 and two similar conical sections 61 which slip over the section of tubing 60 and are welded together centrally of the structure. A cover plate 72 is fitted against a shoulder formed internally of the upper end of the housing 10 and may be welded in place.

The desirability of slotting the openings in the cathode and end wall support openings to prevent entrapment of gas has been noted above. In this embodiment the slots and 82 in the cathode ends and the slots and 92 in the end wall openings are illustrative of this feature. Electrical connections are made to the anode by means of a lug 55 to which the anode conductor 32 is united. Because anode support is derived from this conductor, a relatively heavy and inflexible connector is preferred. If the gauge is to be used in environments where shock or vibration are serious problems, an opening or indentation may be formed in the anode opposite the heavy conductor, and another opening or indentation may be formed in the housing inner wall to permit the use of ball support of the type disclosed for the cathode.

Connection to the cathode is accomplished directly by the welding of a relatively long lead 76 directly to the outwardly extending flange of the conical portion of the cathode of the device. The lead 76, of course, also constitutes the central conductor of the cable end seal 60.

The principles of operation and general assembly method in the embodiments of FIGS. 3, 4 and 5 are similar to those of FIGS. 1 and 2, although features which are found in the devices of FIGS. 1 and 2 are not as fully realized in the latter embodiment.

However, support of. the cathode by compression between the end walls 72 and 73 of the housing is a feature of the invention which is retained. 'In this instance, the ceramic spheres 64 and 57 are disposed between openings found in each end wall and the ends of the cathode tube 60. The openings are of smaller diameter than the spheres, and welding of the top plate 72 and the bottom plate 73 in place locks the cathode in position under compression. Insofar as electrical connections are concerned, they are essentially the same in both embodiments of the invention shown and described. As is indicated in FIG. 2, the housing or envelope 10 is preferably grounded, a source of high voltage is connected to the anode through the stud 35 and an ion current detector is connected to the cathode through the stud 39.

There are numerous advantages that may be realized from using cold cathode ionization gauges rather than older hot cathode devices. Most important is the extension of the range of possible pressure measurements to previously unattainable low levels. Measurements of very low pressures are inherently unobtainable where heated metallic elements and oxides are used to produce or enhance thermionic emission principally because of filament vapor pressure. Also, ultraviolet radiation from a heated filament can produce photoelectric emission from the ion collector as occurs as X-rays are produced when electrons strike the positive grid of a hot filament ionization gauge. In the cold cathode gauge, the production of X-rays decreases with gas pressure; and furthermore, a magnetic field used with a magnetron type of cold cathode gauge minimizes the escape of photoelectrons from the ion collector.

Understand-ing of the present invention may be further facilitated by a brief consideration of general operational features of a magnetron type of cold cathode gauge. A magnet produces a uniform D.C. magnetic field along the axis of the gauge as indicated by the line H'H in FIGS. 2 and 5. The envelope is grounded, a high voltage is connected to the anode and the current flow to the cathode is measured with a sensitive electrometer to provide the desired measurement of gas pressure, all as indicated in FIG. 2. Suitable return connections to ground are made from the electrometer and high voltage supply.

At high pressures, discharge occurs almost immediately upon the application of high voltage, the grounding of the envelope aiding discharge initiation, and a steady current begins to flow. When, however, the pressure is reduced to 10- torr or below, discharge may be delayed for a period of time after the application of high voltage. The discharge is believed to be a self-sustained Townsend discharge. It is theorized that the discharge begins with a relatively small number of initial electrons attempting to move more or less radially in the direc* tion of the electric field which is produced. In the presence of a strong magnetic field, the electrons follow a cycloidal path and initially, they have collisions with gas molecules. With each collision, a positive ion and a new electron result. Under such conditions, a multiplication of electrons ensues.

However, the electric and magnetic fields in the gauge are such that positive ions can move freely to the cathode at sufficiently low gas pressures. The electrons, on the other hand, are trapped radially by the magnetic field and cannot move freely to the anode. In this situation, secondary electrons .are produced by positive ions striking the cathode with sufficient energy. Additional secondary electrons are produced by the inelastic collisions between electrons and the gas molecules in the gauge, which collisions produce excited atoms and molecules. The excited gas atoms and molecules, in turn, produce photons which release these additional secondary electrons upon their striking the cathode. The discharge becomes self-maintained as the number of electrons and ions being created equals the number being lost per unit time. Thus, a cloud or swarm of electrons are in motion between the anode and the cathode of the gauge, and its density does not vary with gas pressure .as long as the electric and magnetic fields are constant.

In both of the illustrated gauges of the present invention, a spool-shaped cathode is utilized. The cathode is positioned with its axis coincident with that of the anode. Centrally, the cathode is a short cylinder of small diameter, but its ends are conical with the cone diameter increasing with distance from the mid-point of the cathode. A magnetic field is parallel to the anode-cathode axis, and the conical cathode surfaces are inclined with respect to that magnetic field. Of course, the electric field between anode and cathode is also modified by the cathode configuration and disposition relative to the anode. The result is that electrons leaving the cathode surface in a direction generally normal to that surface have components of velocity that are both parallel to the magnetic field and transverse to the magnetic field. The trajectories of such electrons are longer than electrons which have only axial or radial motion.

Generally, an electron leaving a purely cylindrical cathode will execute a single arc of a cycloid and return to the cathode surface to be collected. No further contribution to the discharge purpose is made by such electrons. An electron leaving a radial cathode whose surface is perpendicular to the magnetic field will move axially in a straight line along the magnetic field direction until it is deflected by an electric field or collected by another radial electrode. In the present gauge, however, surfaces of significant area are inclined to .the direction of the magnetic field.

In the embodiments of the invention shown and described, the inclined surfaces are the conical ends of the cathode. However, other configurations are quite feasible, the only basic requirement being that at least a part of the cathode surface be so inclined. An electron leaving such an inclined surface in a normal direction does not immediately return to that surface, and its path is greatly lengthened before it strikes another surface. The lengthened path of the electron is principally of value because it increases the sensitivity of the ionization gauge. Furthermore, the linearity of the gauge is improved by such electron behavior, and discharge within the gauge is more easily started simply because the loss of electrons is minimized.

It has previously been mentioned that stainless steel is a suitable material from which the gauge may be fabricated. Stainless steel is not necessarily the best material, but it is readily available. Other metals such as nickel, Inconel and Nichrome V have also been successfully employed. It is desirable to fire the metallic components in hydrogen or vacuum to temperatures of about 800 C. before they are assembled in order that outgassing of the components during bake-out and later use be minimized.

The supporting balls or spheres which are preferred for use within the gauge are of high density alumina. They are preferably manufactured to very close tolerances and with extremely smooth surface finishes. Because they retain their hardness at greatly elevated temperatures and have a low coefficient of expansion, they are particularly valuable in the present environment. However, other insulating materials such as ceramic, sapphire, quartz, glass or Mycalex may be used.

The sealing material used by the conductors and tubular extensions of the gauge is also preferably high density alumina. With a glazed surface, a very high surface resistivity is achieved. A permanent magnet made of Alnico V, but having pole pieces of low carbon steel provides the needed magnetic field. It is arranged about the gauge in such a manner that the air gap exists between the top and bottom flat surfaces of the gauge, and it may phovide a magnetic field strength of about 1100 oersteds, which is quite sufficient. Other magnetic materials are also suitable. For example, Alnico VI has been used to provide magnetic field strength of about 900-950 oersteds, and operation is excellent. in this connection, it should also be pointed out that the gauge is usually of the order of one and one-quarter inches in thickness, which indicates the length of the air gap.

The rather large flange which is provided on one of the gauges disclosed serves a definite purpose. Enough metal is provided in the flange to permit the top cover to be machined off a number of times to permit repair or cleaning of the interior of the gauge.

The envelope of the gauge being metal and grounded eliminates the possibility of the electric field becoming distorted by the presence of a charged surface, as is sometimes encountered in glass envelopes. Too, electrical surf-ace leakage currents from anode to cathode are not possible because of .the grounding of the envelope. The opacity of the metal shields the contents from ultraviolet and soft X-ray radiation as well as from external electrical disturbances.

The fact that the anode is so constructed and supported that only the aperture opposite the tubulation communicates with the discharge region between anode and cathode prevents outgassing products from reaching that region, in addition to aiding speed of response as noted above. When indentations are used, the support spheres are completely out of the discharge area, and only a relatively small area of the support spheres are even near the discharge region when apertures are used. However, they are so recessed as to cause no difiiculties such as the inner portions of the flat ceramic rings of prior art structures have caused when coated with sputtered metal.

Although what has been disclosed constitute preferred embodiments of the invention, the dis-closure is only illustrative of the invention as it may be practiced successfully. Given the foregoing disclosure as a guide, those skilled in the art will have no difficulty in making minor modifications and alternatives which fall within the purview of the invention. Hence, the invention should not be limited to the details shown and described, but only by the spirit and scope of the appended claims.

What is claimed is:

1. In a magnetron-type cold cathode ionization gauge having a magnetic field of a given direction passing there-through, a generally cylindrical metallic housing, a tubular anode spaced from and supported under compression by the wall of said housing, top and bottom members completing the enclosure of said cylindrical housing, a cathode disposed coaxially within said anode, said cathode being spaced from and supported under compression by said top and bottom members.

2. In a magnetron-type cold cathode ionization gauge as defined in claim 1, the further combination therewith of first support means under compression between said wall of said cylindrical housing and said anode and second support means under compression between said top and bottom members and said cathode.

3. In a magnetron-type cold cathode ionization gauge, the combination of a metallic envelope of generally cylindrical configuration having parallel top and bottom members, said envelope having a peripheral channel formed in the interior Wall thereof, said top and bottom members having substantially central recesses formed in the inner surfaces thereof, a tubular anode, first insulating support members for said anode compressed between said envelope and said anode, said support members being disposed in said peripheral channel, a cathode disposed within and coaxially of said anode, and second insulating support members disposed in said central recesses of said top and bottom members, said second insulating support members being compressed between said cathode and said top and bottom members.

4. In a magnetron-type cold cathode ionization gauge as disclosed in claim 3, a tubular anode having recesses formed therein, and a cathode having open circular ends, said first insulating support members comprising spheres of insulating material disposed in and compressed between said peripheral channel and said recesses in said anode, and said second insulating support members comprising spheres of insulating material disposed in and compressed between said cathode circular ends and said central recesses in said top and bottom members.

5. In an ionization gauge having a metallic envelope, the combination of an anode, a cathode and supports for said anode and cathode, said supports comprising insulating spheres compressed between the inner Wall of said envelope and said anode and cathode.

6. In :an ionization gauge, a cylindrical envelope having an internal peripheral channel formed therein, an anode having a plurality of recesses formed therein, and a plurality of spheres lodged in said channel and in said apertures to lock said anode in position relative to said envelope.

7. In a magnetron-type cold cathode ionization gauge, the combination of ame-t-allic envelope of generally cylindrical configuration having parallel top and bottom members, said envelope having a peripheral channel formed in the interior Wall thereof, said top and bottom members having substantially central recesses formed in the inner surfaces, thereof, a tubular anode, first insulating support members for said anode compressed between said envelope and said anode, said support members being disposed in said peripheral channel, a cathode disposed within and coaxially of said anode, at least a central recess being formed in the top of said cathode and at least a central recess being formed in the bottom of said cathode, a first insulating sphere being compressed between said top member of said envelope and said central recess formed in said top of said cathode and a second insulating sphere being compressed between said bottom member of said housing and said central recess formed in said bottom of said cathode whereby said cathode is supported centrally of said housing.

8. In an ionization gauge, an envelope having top and bottom members, said top and bottom members having similar recesses formed therein, a cathode having circular openings formed in the ends thereof and a pair of spheres lodged between the openings in said cathode and the openings in said top and bottom members to support said cathode centrally of said envelope, a generally tubular anode spaced from and surrounding said cathode, and an anode conductor connected to said anode and supported in insulating relationship from said envelope.

9. An ionization gauge comprising a metal housing, a cathode disposed in said housing, spherical insulating supports disposed between said housing and said cathode for holding said cathode in position in said housing, a tubular anode surrounding said cathode, and an anode lead sealed in insulating relationship to said housing and connected to said anode to provide support thereto.

10. An ionization gauge comprising a metal housing, a cathode disposed in said housing, spherical insulating supports disposed between said housing and said cathode for holding said cathode in position in said housing, a tubular anode surrounding said cathode, an anode lead sealed in insulating relationship to said housing and connected to said .anode, and a plurality of spherical insulating supports disposed between said anode and said hous- 9 10 ing, said anode being supported by said anode lead 'and- 2,916,649 12/1959 Levin 31329 2 X said spherical insulating supports. 3,051,868 8/ 1962 Redhead 3137.5

References Cited by the Examiner FOREIGN TE UNITED STATES PATENTS 5 754,515 8/ 1956 Great Blltalll.

,6 1/ 1947 Chess 6t 81 X JAMES W. LAWRENCE, Primary Examiner. 2,431,887 12/1947 Penning 313157 X 2,509,951 5/1950 Anderson 313-292 X GEORGE WESTBY DAVID 2,754,349 7/1956 Werner 313 292 X 2,884,550 4/1959 Lafferty 313157 X 10 R. JUDD, Assistant Examiner. 

1. IN A MAGNETRON-TYPE COLD CATHODE IONIZATION GAUGE HAVING A MAGNETIC FIELD OF A GIVEN DIRECTION PASSING THERE-THROUGH, A GENERALLY CYLINDRICAL METALLIC HOUSING, A TUBULAR ANODE SPACED FROM AND SUPPORTED UNDER COMPRESSION BY THE WALL OF SAID HOUSING, TOP AND BOTTOM MEMBERS COMPLETING THE ENCLOSURE OF SAID CYLINDRICAL HOUSING , A CATHODE DISPOSED COAXIALLY WITHIN SAID ANODE, SAID CATHODE BEING SPACED FROM AND SUPPORTED UNDER COMPRESSION BY SAID TOP AND BOTTOM MEMBERS. 